The optimum high pressure for CO2 transcritical refrigeration systems with internal heat exchangers

The system performance of a CO2 refrigeration system is greatly affected by the compressor discharge pressure. An internal heat exchanger (IHX) with high effectiveness is an important factor to achieve high system performance. The expression traditionally used to describe the heat exchange effectiveness is not suitable for CO2 systems. As a result a practical effectiveness expression for IHX, based on enthalpy difference, has been derived and is reported in this paper. Detailed analysis on the relationship between the optimum high pressure Pk,opt and other systematic parameters was performed. Evaporating temperature has little influence on Pk,opt; and IHX can minimize the sensitivity of the system to the refrigerant quality x at the evaporator outlet. Based on simulation data, a correlation of Pk,opt was developed that predicts the simulation values with a deviation of less than 3.6% in the whole range and 0.94% when the evaporating temperature t1=5.3 °C. The results reported in this paper can be used in optimum control and performance evaluation of the whole system.